This invention relates to a solid-state laser apparatus for exciting a solid-state laser medium placed inside a resonator with a beam of pump light from an appropriate source and thereby causing a beam of laser light to be emitted from the resonator.
In order that the transverse mode of the laser beam generated by such an apparatus be a single transverse mode (TEM.sub.00), it has been know to make the pump light incident such that its transverse (pumping) mode coincides with the resonator mode of the solid-state laser medium or is included in the resonator mode. It is not a simple matter, however, to focus the pump light such that all of its pumping modes will be incident within the resonator mode in the solid-state laser medium because a complicated optical system will be required for the purpose. A particularly complicated optical system will be necessary if a semiconductor laser element is used as the source of pump light for the solid-state laser apparatus because the transverse mode of laser light from a semiconductor laser element is an elongated ellipse either because the stripe of active layer of a semiconductor laser element for emitting laser light is about 1.mu.m in thickness while its width is about 100 .mu.m or due to astigmatism. Thus, a complicated optical system using a complicated lens such as an anamorphic prism pair or a cylindrical lens or a large number of lenses is required in order to change this shape into a true circle and to focus it within an area with a smaller diameter than that of the resonator mode. In order to obtain a high-power emission of solid-state laser light, the width of the stripe of the active layer of the semiconductor laser element must be increased with respect to its thickness, but this will make the transverse mode of the emitted laser light much flatter, and an even more complicated optical system will be necessary. If a complicated optical system is incorporated, not only the size of the solid-state laser apparatus itself but also the energy loss of the pump light will become large, making it economically disadvantageous.
In the case of a quasi-three-level laser system, furthermore, there is the big problem of reabsorption, or the absorption by the laser medium of the fluorescent light generated by spontaneous emission when the electrons excited by the pump light drop to lower energy levels, as well as the laser light which has just come to be emitted. Unless the pump light remains incident on the resonator mode in the solid-state laser medium, the reabsorbed energy is converted into heat, thereby raising the threshold for the laser light emission.